Conventionally, a RAID (Redundant Array of Inexpensive Disks), in which a plurality of small disks are arrayed to improve the performance and the reliability of the disk system and to reduce the cost, has been used to provide a high reliable disk system.
Various configurations are proposed to realize a RAID disk array. In some of them, data stored in a single disk is fragmentized and is circularly stored in a plurality of disks. Thus, for large volume data, plural disks transfer data at one time to improve the transfer speed depending on the number of disks. For small capacity data, the data distributed to each disk is independently accessed to increase throughput. When trouble occurs, information on a defective disk which causes the trouble can be restored from the information on an effective disk by use of a redundant disk, so that the reliability can be obtained. Alternatively, the redundancy may be easily preserved by duplicating a disk.
A unit price of a small capacity disk for a unit capacity is lower than that of a high quality large capacity disk due to mass-production. Therefore, the price of the entirety of the disk array in which a plurality of small disks are arrayed can be reduced.
To realize a disk array having the above-described benefits, in which a plurality of small disks are arranged, there is a variety of RAID techniques. Other than the RAID techniques, a striping configuration can, alternatively, be used.
In the striping configuration, two or more HDDs are combined to construct an apparent large capacity drive. In RAID configurations, there are RAID 1 in which two HDDs are configured in a mirroring configuration, RAID 2 having a redundant disk for recording a Hamming code in addition to a plurality of HDDS, RAID 3 in which a parity disk for redundancy is used, RAID 4 in which the striping unit is defined by a block and RAID 5 in which data and parity information to reconstruct the data are distributed and written.
These disk array configurations will be described hereinafter in detail with reference to FIGS. 10a to 10f. 
In these RAID configurations, if it is required to expand the capacity, a (n+1)-th disk is added to a RAID system of n disks, and the additional disk is incorporated in the existing RAID configuration, so that the system capacity is expanded. However, in this expanding method, if a physical factor, i.e., if a slot into which a disk is inserted is not provided, a new disk cannot be inserted into the RAID configuration. In such case, the capacity cannot be expanded. Namely, in such disk array apparatus, no expansion of capacity is possible.
Nevertheless, even in the disk array apparatus, it is sometimes required to expand the storage capacity. Therefore, in order to meet the requirement of storage capacity expansion, a new disk array apparatus having an increased capacity must be prepared. Additionally, in order to replace the disk array apparatus, an operating RAID-stored data must be once saved in a magnetic tape medium or the like, and the data must be restored from the saving medium after a new RAID configuration is constructed.
As described above, a large capacity recording apparatus such as a magnetic tape apparatus or the like is required, and not only is a process of the construction of a RAID configuration complicated but also a computer system must be stopped during reconstruction that requires long time. Thus, the operation of the computer is hindered.